The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 4,177,946; 4,285,472; 4,430,020; 4,473,191; 4,534,515; 4,874,132; 5,106,021; 5,615,838; 6,027,048; 6,206,305 and 6,382,530.